9 Deep sea and Offshore/Pelagic Habitats David Obura Opposite page: Larval squid in the plankton of the WIO. © Cedric Guiguand/Emmanuel G. Reynaud. INTRODUCTION • India split from Madagascar and moved northwards, leaving behind a fragment which is now the North Sey- The deep sea habitats of the Western Indian Ocean (WIO) chelles Bank 80 mya. are very poorly known, particularly from the perspective of the governments in the region, which have limited capacity The WIO floor is composed of two major plates, the to engage in deep sea research or plan for exploitation of African and Indian plates; the Australian plate lies to the resources in this environment in the near future. Neverthe- east, the Arabian plate to the north. The ocean floor is still less, there are some emerging exceptions to this which will tectonically active, with a spreading rift along the South be presented in this chapter. In view of the paucity of general West and Central Indian Ocean ridges, and northwards in knowledge on deep sea habitats in the region, this introduc- the Carlsberg ridge that extends into the Red Sea. This tion includes information on their basic structure, followed in continuous ridge system forms the approximate eastern later sections by a summary of their status, knowledge on boundary of the WIO and the African coastline defines the their biodiversity, and pressures/trends in the future. western boundary (ASCLME/SWIOFP 2012a, ASCLME/ SWIOFP 2012b, Figure 9.1). Plate tectonics The WIO contains both active and fossil tectonic plate Hotspots and other geological features margins, some of the deepest fracture zones in the world, Within its broader tectonic margins, the WIO contains a the most complex mid-ocean ridge configurations and number of active hotspots, two of which are of global sig- some of the thickest sedimentary sequences in the world’s nificance and scale. The Mascarene-Reunion hotspot ocean basins. The continental land mass of Africa, Mada- became active at the Cretaceous-Tertiary (K-T) boundary gascar, and the North Seychelles Bank are remnants of the about 67-64 mya, through a massive eruption of magma supercontinent Gondwana, which dates from pre-Cam- that formed the Deccan Traps in India (classified as a Large brian times over 650 million years ago (mya), and that Igneous Province, or LIP). As the Indian plate moved started to break up 180 mya. Key events include: northwards over the hotspot, a series of island chains were • Formation of the Mozambique Channel, from about formed: Lakshadweep – Maldives (57-60 mya), the Chagos 180 -150 mya when Madagascar, Australia, India and Ant- Archipelago (48 mya), Saya de Malha (45 mya), Nazareth arctica rifted from the African coast. and Cargados Carajos (34 mya), Mauritius (7-8 mya) and • Australia-Antarctica started to separate from Mada- Reunion (0-2 mya). Only the youngest two, Mauritius and gascar-India 120 mya. Reunion have volcanic features breaking the surface (the Western Indian Ocean 111 FinalRESOCR.indd 111 15/09/16 13:06 III . Assessment of marine biological diversity and habitats Figure 9.1. Deep sea features of the Western Indian Ocean, including principal basins and currents Bathymetry is shown at 100 m depth (light blue shading) then at 500 m and 500 m intervals thereafter. Abbreviations: AC Agulhas Current, EACC East Africa Coastal Current, EMC East Madagascar Current, MCE/CG Mozambique Channel Eddies/Comoros Gyre, SC Somali Current, SEC South Equatorial Current. latter is still volcanically active today); all the others have Ocean basins sunken below the ocean surface and are capped by bio- As a result of these various geological features, the WIO is genic carbonate platforms and coral atolls or islands. The split into four deep basins: the Madagascar (5 500 m, south- Central Indian Ocean ridge itself moved over the hotspot east of Madagascar), Mascarene (4 900 m, west of the Mas- about 45 mya, separating the Chagos and Saya de Malha carene Plateau), Mozambique (5 000 m, south of the Banks. During the Cretaceous, the continental fragment of Mozambique Channel) and Somali Basins (5 100 m, Madagascar lay over the Marion hotspot, creating a subma- between Somalia and the Seychelles, Figure 9.1). Little rine Plateau which extends southwards some 1 300 km at work has been done on the abyssal plains and soft sedi- depths of 1 000-2 000 m, rising above the deeper basins to ments of these basins, though drilling on and near the Mas- the east and west up to 5 000 m deep. carene Plateau shows thick accumulations of marine Seamounts in the WIO are concentrated on the mid- sediments, and terrestrial sediments in the Mozambique ocean ridges, particularly the South West Indian Ridge, Channel, probably washed off the African and Madagascan and scattered around the Mascarene Plateau. Approxi- landmasses by large rivers over >100 million years. mately 700 seamounts have been identified in the WIO region from global studies of bathymetry ASCLME/SWI- Oceanography OFP 2012a, ASCLME/SWIOFP 2012b). Most seamounts The oceanography of the Western Indian Ocean is deter- in the WIO are outside national jurisdiction, with only mined by interactions between the geological features out- those scattered around the Mascarene Plateau being within lined above with the equatorial and western boundary the EEZs of the regional countries. A complex geological currents of the ocean basin. The South Equatorial Current history is suggested by the multiple islands and island (SEC) enters the WIO as a broad slow surface current groups within the Seychelles-Mascarene region (eg Aldabra stretching from about 5-16°S, fed from the Indonesian group, Farquhar/Providence and the Amirantes). Other Through-Flow with waters from the Pacific Ocean, and volcanic features include the island of Rodrigues and passing the Chagos Archipelago at its northern edge. At the Soudan Bank, seamounts in the Seychelles that break the Mascarene Plateau, the SEC is partially blocked, with 50 surface as coral reefs, the Comoros Archipelago, the Iles per cent of its flow forced through the narrow gap between Eparses, and the Davies Ridge in the Mozambique Chan- the Saya de Malha and Nazareth Banks at about 12°S, the nel. remainder flowing both north and south of these two banks. 112 Regional State of the Coast Report FinalRESOCR.indd 112 15/09/16 13:06 9. Deep sea and offshore/pelagic habitats Approaching Madagascar, the main flow of the current is at that returns water from the East African coast towards the about 17°S. The banks and island systems lying in the Seychelles, at about 0-2°S. The Asian landmass thus drives pathway of the SEC function as stepping stones for species the seasonal monsoon system that dominates climate in crossing the Indian Ocean, providing genetic connectivity. the central and northern parts of the region. Seasonality in Interestingly, there is evidence pointing to west-east trans- the NECC contributes to west-east connectivity that port (Sheppard and others, 2012) as well as in the expected apparently passes through the Chagos/Maldives/Lakshad- east-west direction. weep systems, returning genetic material to the Eastern The tip of Madagascar and the Comoros-Glorieuses Indian Ocean. chain interact with the flow of the SEC and open ocean features such as Rossby waves to generate unique and Productivity highly dynamic meso-scale eddies within the northern Biological diversity patterns in the WIO are driven by the Mozambique Channel. The “Glorioso Front” probably geological and oceanographic features described above. marks the transition from the SEC to the waters of the Interactions between water flow and shallow or deep channel, where a series of clockwise and anti-clockwise bathymetric features can result in strong downstream eddies and an intermittent gyre around the Comoros chain dynamics as a result of eddies (mixing and turbulence) are induced (Ternon and others, 2014). Being driven by induced in the wake of the features, with raised productiv- these features, water may flow in any direction, resulting in ity resulting from this and from the addition of nutrients a highly mixed and dynamic water body in the northern and minerals to otherwise oligotrophic and mineral-poor part of the channel. As the channel narrows at about 17°S, oceanic waters. Closed circulation cells may also form the flow becomes more southward just offshore of the around seamounts, inducing upwelling, trapping nutrients Mozambique coast. Complex forcing of biological parame- and enhancing primary productivity (Harris 2011, Keating ters results from these dynamics, including up- and down- and others, 1987). welling in the eddies and their interactions with the The seasonal dynamics of the Somali Current strongly continental shelves and slopes below at least 1 000 m influence productivity in the northern part of the WIO depth. As a result, the Mozambique Channel is one of the (Schott and McCreary 2001), with strong upwelling in May most energetic western boundary zones of all the world’s to September along certain parts of the coast due to the oceans (Ternon and others, 2014). northward-flowing East African Coastal Current. The sea- The rapid flow of the East Madagascar Current inter- sonal upwelling shuts down during the northeast monsoon acts with the Madagascar Plateau, which extends south- in December to April, but complex currents redistribute wards over 1 000 km at depths of 1 000 to 2 000 m, as it biological production from the upwelling events through- curls around the southern tip of Madagascar. This results out the northern Indian Ocean. in highly dynamic nearshore eddies and nearshore–off- Upwelling in the open ocean between 5-10°S in the shore upwelling over 100s of square kilometres of sea, Central and Western Indian Ocean is indicated by meso- enriching highly productive food webs.